The present invention relates to a noise reduction system provided in a sound reproducing system.
A tape deck is provided with a noise reduction system wherein the level of sounds in middle to high frequency range where tape hiss is most objectionable, is compressed at recording and expanded at reproduction of the sounds.
In the noise reduction system, the music signals must be controlled both at recording and at reproduction. In addition, the noise reduction cannot be carried out at the reproduction unless the signals are compressed at the recording.
In consideration to these problems, the applicant of the present patent application has proposed an effective noise reduction system in Japanese Patent Application No. 8-139535. In the system, a signal reproduced from a recording medium is divided into frequency bands of a predetermined number. In each frequency band, the sound level of the reproduced signal is compared with a predetermined threshold, which corresponds to a noise level in music signals. When the level of the reproduced signal is lower than the threshold, the reproduced signal is deemed a noise, not a music signal. Therefore the reproduced signal is removed. The divided reproduced signal is thus processed in each frequency band, and thereafter, added together so as to be outputted as the music signal.
Although it is possible to use the noise reduction system at a recording of the music signal on a recording medium, there occurs the following problems. Namely, the level of noise component, and the distribution of the noise component in a frequency range largely differ dependent on the recording level, that is, input level of the signal, and the adjusted level of volume at recording, and also on the type of the music signal which is to be recorded. In addition, if the music signal source is a tape deck, the method for reducing the noises at recording further affects the level of the distribution of the noise components. In case of a tuner, the noise level correspondingly changes as the receiving conditions change.
Hence, when only one threshold is used, the noise reduction system cannot respond to various changes in conditions, so that the noise cannot be sufficiently reduced. Furthermore, the noise level fluctuates as in double breezing noise, thereby causing auditory problems.
Moreover, the frequency response of the noise differs depending on the type of the music signal. When the noises in all kinds of music signals are reduced in accordance with the threshold for a uniform frequency response, the problems described above also occur.